1. Field of the Invention
The present invention relates to an image forming apparatus that has a function of forcibly moving an abutting body away from an image carrier by driving a rotating cam, the abutting body being capable of abutting the image carrier by means of a biasing force of a biasing part to form a transfer nip.
2. Description of the Related Art
Use of a thick sheet as a recording sheet in this type of image forming apparatus often causes a linear unevenness in the density of the image, which is called “shock jitter.” When the thick sheet enters a transfer nip, the burden on an image carrier drastically increases and the linear speed of the image carrier is significantly reduced instantly, causing this density unevenness.
The image forming apparatus described in Japanese Patent Application Laid-open No. H10-83124 inhibits the occurrence of the shock jitter as follows. In this image forming apparatus, a transfer roller functioning as an abutting body has a cylindrical roller part, and a shaft member that projects from each end surface of the roller part to rotate integrally with the roller part. The shaft member on each end surface is provided with a rotating cam spinnably. These rotating cams are directly coupled to a motor for spinning the rotating cams on circumferential surfaces of the shaft members. The rotating cams that are spun on the circumferential surfaces of the shaft members by the motor have convex parts thereof brought into abutment with an end part of a photoreceptor, the image carrier, in its shaft direction at a predetermined rotational angle position. This abutment forcibly moves the transfer roller, which is biased toward the photoreceptor by a spring, away from the photoreceptor against a biasing force, so that the inter-shaft distance between the photoreceptor and the transfer roller can be increased. When a thick sheet is used as a recording sheet, the abovementioned inter-shaft distance is increased to reduce the nip pressure by forcibly moving the transfer roller or the transfer roller is separated from the photoreceptor. In this manner, the occurrence of the shock jitter can be inhibited by preventing a drastic increase in the burden on the photoreceptor, which occurs at the time of entry of a thick sheet. However, the increase of the abovementioned inter-shaft distance might cause a transfer failure due to a lack of transfer pressure.
The image forming apparatus described in Japanese Patent Application Laid-open No. H6-274051 is known as an image forming apparatus capable of avoiding the occurrence of such transfer failure. In this image forming apparatus, prior to the entry of a thick sheet into a transfer nip, a transfer roller is separated from a photoreceptor by driving a rotating cam, to form a tiny gap between the transfer roller and the photoreceptor. In this manner, the occurrence of the shock jitter is prevented. Next, immediately after allowing the thick sheet to enter the abovementioned tiny gap, the rotating cam is driven to release the forced movement of the transfer roller, and the transfer roller is pressed against the photoreceptor by taking advantage of the biasing force of the spring. As a result, sufficient transfer pressure can be exercised during the transfer process to prevent the generation of a transfer failure.
However, the problem of this configuration is that the convex parts of the rotating cams are worn out in a short period of time by frequently driving the rotating cam, significantly reducing the life of the rotating cams.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Application Laid-open No. H4-242276.